Adjusting Flood Peak Frequency Changes to Account for Climate Change Impacts in the Western United States

Abstract

One consistent projection for the western United States has been for increasing peak streamflow as the global climate warms. Although past studies have characterized some aspects of future streamflow projections, this effort exploits new data sources to estimate changing peak flow frequency based on output from many climate projections, which drive a physically-based hydrology model. Using historic and projected future streamflow simulations at 421 sites across the western United States, changes in peak flows for common recurrence intervals are estimated. A parameter relating the recurrence interval needed for design in the present to produce a future required recurrence interval is derived and mapped across the United States, illustrating a method for adapting design to a changing hydrologic setting. For this demonstration, using a higher business-as-usual greenhouse gas emissions pathway, peak flow increases were proportionately larger for the more rare 1-year (or 1% exceedance) event than the 1-year event. Compared to 1971–2, the domain-wide peak flow magnitude is projected to increase by 14–19% for early 21st century and 31–43% by the end of the 21st century, depending on recurrence interval. Impacts under lower emissions pathways will be more modest (or occur further in the future). In terms of return period, by the end of the 21st century, the 1-year event of the late twentieth century is projected to be approximately a 4-year event, representing a 2.5-fold increase in occurrence probability. This approach offers a strategy for regional planners to incorporate these projected changes into design based on flood flow frequency.